Conventionally, some diesel engines have selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flows, said catalyst having a feature of selectively reacting NOx with a reducing agent even in the presence of oxygen; a required amount of reducing agent is added upstream of the catalyst to be reacted with NOx (nitrogen oxides) in exhaust gas on the catalyst to thereby reduce a concentration of the discharged NOx.
Meanwhile, effectiveness of ammonia (NH3) used as a reducing agent for reduction and purification of NOx is well known in a field of industrial flue gas denitration, for example, in a plant. However in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, researches have been made nowadays on use of nontoxic urea water as the reducing agent (see, for example, Reference 1).
[Reference 1] JP 2002-161732A
More specifically addition of the urea water to the exhaust gas upstream of the selective reduction catalyst under a temperature condition of about 170-180° C. or more causes the urea water to be decomposed into ammonia and carbon monoxide, and NOx in the exhaust gas or the catalyst is satisfactorily reduced and purified by ammonia.
In such exhaust emission control device, it is important to properly and adequately control an injection amount of the urea water to keep the NOx reduction ratio as high as possible. However, reaction rate of NOx reduction on the catalyst greatly depends upon temperatures of the catalyst, so that it necessary to correct a basic injection amount determined on the basis of a current engine operation status so as to be consistent with the reaction rate by using entry or exit temperature of the catalyst as substitution for the catalyst temperatures.